The present invention relates to an improved device for enabling helium to be transferred at a very low temperature between non-rotating member of a refrigerating machine and a rotating member, such as the rotor of a rotating electrical machine having a superconducting rotor winding.
In rotating electrical machines having superconducting windings, devices for the transfer of helium typically have to be able to inject liquid helium at a very low temperature (4.2.degree. K) into a cryostat containing a superconducting winding to cool the winding. The devices also have to be able to extract the gaseous helium from the cryostat after the helium has become heated from being in contact with the superconducting winding. Finally, the devices are used to return the gaseous helium to the refrigerating machine to be reliquified.
In rotating electrical machines having superconducting windings which are the least advanced on thermodynamic and thermal planes, the gaseous helium is extracted at an ambient temperature by the transfer device. Such an arrangement requires the use of a refrigerating machine having great power and consuming a great deal of energy because it has to be capable of causing the helium to pass from the gaseous state (at the ambient temperature) to the liquid state (at a temperature of about 4.2.degree. K.).
In machines which are most highly developed on thermodynamic and thermal planes, the helium transfer device includes an outlet for gaseous helium at a very low temperature (of the order of 6.degree. K.) and an outlet for gaseous helium at ambient temperature. The latter outlet is intended to return to the refrigerating machine the gaseous helium used to cool the current supply lines leading to the inductor and the suspension members. In this arrangement, the refrigerating machine can have a much lower power rating because a greater proportion of the helium which is cooled by the refrigerating machine so as to be liquified is already at a very low temperature.
The necessary presence of a refrigerating machine relatively close to a rotating electrical machine having superconducting windings, and the necessity of enclosing the superconducting windings in cryostats which are responsible for thermal insulation, limits the application of superconductors to heavy torque machines such as turbo-alternators and naval propulsion motors. The reliability demanded from these machines is very high. A failure of the helium transfer device in these machines would risk causing the machine to be out of action for a long time. As is presently known, the device for the transfer of helium is generally mounted at the end of a shaft of the machine which does not transmit mechanical torque, and therefore cannot be duplicated.
In the simplest case, that is to say when it is a question of supplying a fixed cryostat with liquid helium, the liquid helium is always conveyed from a liquid helium reservoir to the fixed cryostat by means of what is called a transfer reed. It is a pipe with two walls between which a high vacuum is maintained. The injection of liquid helium into a rotating cryostat is effected in a similar manner but using two coaxial transfer reeds, namely a rotating transfer reed mechanically connected to the rotor of the rotating electrical machine and a fixed transfer reed connected to the liquid helium reservoir and partially engaged in the rotating transfer reed. An escape of helium inevitably occurs in the overlapping region of the two transfer reeds and passes progressively from the temperature of the liquid helium to the ambient temperature. In order to limit this escape and to avoid convection movements in the gaseous helium, only a slight radial play is tolerated between the fixed and rotating transfer reeds.
In the more advanced machines in which the greater portion of the gaseous helium is extracted at low temperature, the extraction of gaseous helium at a very low temperature may also be effected by means of two transfer reeds respectively fixed and rotating, partially engaged one in the other and concentric with the transfer reeds for the injection of the liquid helium. The two transfer reeds for the extraction of the gaseous helium at a very low temperature are thus subjected to similar conditions with regard to their relative radial play in their overlapping region.
The extraction of gaseous helium at ambient temperature generally does not present any problem and may be effected concentrically and outside the transfer reeds for the injection of liquid helium and, where appropriate, outside the transfer reeds for the extraction of gaseous helium at a very low temperature.
As seen above, the radial clearances between the fixed reed or reeds and the rotating reed or reeds are very slight. The end of the shaft of the rotating electrical machine, where the transfer device is located, may vibrate intensely with the result that friction may occur between the fixed reed or reeds and the rotating reed or reeds. In the long run, such friction may lead to perforation of the one reed or of the other and consequently lead to a loss of vacuum in the vacuum compartment of the transfer reed which has been perforated. Such an incident would cause a very long stoppage of the machine.
The essential object of the present invention is to avoid perforation of the transfer reeds as a result of friction between them and consequently to increase the reliability of the helium transfer device.
For this purpose, the present invention provides a device for the transfer of helium between a fixed refrigerating machine and a rotating cryostat of a rotating electrical machine having a superconducting rotor winding. The device includes two coaxial transfer reeds for the supply of liquid helium, with one of the two supply reeds rotating with the rotating cryostat and the other supply reed being relatively fixed. The fixed reed is partially engaged in the rotating supply reed with a slight radial clearance provided in their overlapping region. A cylindrical sleeve which is mounted for rotation with the rotating supply reed extends axially between the two supply reeds, substantially over the whole axial extent of their overlapping region.